Tubular heat transfer members

ABSTRACT

A vertical heat transfer tube for condensation of vapor in a condenser is of substantially uniform wall thickness and grooved so as to impart turbulence to coolant passing through the tube and to assist in runoff of condensate on the tube surface. The hand of the part-helical lands and grooves changes at intervals along the tube before the helix has completed a complete turn. The pitch of the grooves (determined by half a wave length of the helix) is more than 0.5 but less than 5 times the inside minimum diameter of the tube.

United States Patent [1 1 Newson et a1.

1 1 TUBULAR HEAT TRANSFER MEMBERS [75] Inventors: lvan Henry Newson;Thomas David Hodgson, both of Didcot, England [73] Assignee: UnitedKingdom Atomic Energy Authority, London. England [22] Filed: Aug. 8,1973 21 App1.No.:386,6l6

Related U.S. Application Data [63] Continuation-in-part of Ser. No,156.713. June 25.

1971, abandoned.

2.864.591 12/1958 Frink 165/177 [451 Apr. 8, 1975 2.396.426 7/1959Ayling 165/184 x 3.212.992 10/1965 Salesse et a1 138/42 X 3.358.74912/1967 Chisholm et a1..... 165/177 X 3,612,175 10/1971 Ford et a1.165/179 3,762,468 10/1973 Newson et a1. 165/110 X FOREIGN PATENTS ORAPPLICATIONS 947,327 1/1964 United Kingdom 165/184 521,548 5/1940 UnitedKingdom 138/38 Primary E.\'aminerAlbert W. Davis. Jr. AssistantExaminerSheld0n Richter Attorney, Agent, or Firm-Larson. Taylor & Hinds[57] ABSTRACT A vertical heat transfer tube for condensation of vapor ina condenser is of substantially uniform wall thickness and grooved so asto impart turbulence to coolant passing through the tube and to assistin runoff of condensate on the tube surface. The hand of the parthclicallands and grooves changes at intervals along the tube before the helixhas completed a complete turn. The pitch of the grooves (determined byhalf a wave length of the helix) is more than 0.5 but less than 5 timesthe inside minimum diameter of the tube.

4 Claims, 5 Drawing Figures TUBULAR HEAT TRANSFER MEMBERS REFERENCE TORELATED APPLICATIONS This application is a continuation-in-part ofcopending application Ser. No. l56,7l3, filed June 25. I971, nowabandoned.

BACKGROUND OF THE INVENTION This invention relates to improvements inheat transfer tubes which are in the main suitable for heat exchange incondensing vapour/non boiling liquid and condensing vapour/evaporatingliquids.

Various proposals have been made hitherto for enhancing the heattransfer between fluids passed along opposite sides of the tube wall butthe special case of a tube for a condensing/evaporating system presentsproblems which are not easily answered. The virtue of a uniform wallthickness tube having a special profile on the tube bore comprisingregular helical grooves with complementary profile on the exterior isrecognised. It has now been appreciated that the different requirementsof the condensing/evaporating functions can be met advantageously bycomplementary profiles of a different nature. This is especially sowhere the tube is employed in the vertical mode and this has a distinctbearing on the optimum shape.

SUMMARY OF THE INVENTION It has now been found that provided the pitchof the grooves in a helically grooved condensing/evaporating tube arewithin a certain range, the dual function of such a tube may beperformed in an advantageous manner by reversing the hand of the helixalong the length of the tube before the helix has completed a completeturn. The preferred range is that the pitch lies between one and fivetimes the inside, ie minimum tube diameter. Pitch on such tube is equalto one half a wave length of the part helical grooves.

Thus, according to the invention a heat transfer eondensing/cvaporatingtube for operation in a vertical attitude has its wall shaped to impartturbulance to the coolant flowing through the tube bore by part helicalparallel grooves and lands in the bore and complementary lands andgrooves along the tube surface to assist in run-off of condensate. thepitch of the grooves being between point live and five times theinternal, i.e., minimum. tube diameter and the hand of the helixchanging along the length of the tube before completing one whole turn.The preferred pitch is preferably about two tube internal or minimumdiameters. The cooling, or non-boiling liquid flows through the tubebore.

The relatively slow helix of the helical grooves gives, on the outsideof the tube, swift run-off of condensate which is desirable whilst,within the tube where the same helix is reproduced, adequate turbulenceis imparted to the coolant by the change in hand of the helix; such aturbulence not being achievable by a continuous helix of that pitch.Within this limitation of tube profile, a further preferred limitationmay be put upon groove depth in that it should depend upon tube inside,ie minimum diameter. The groove depth should be between 0.02 and 0.2,and preferably about 0.15, times, the tube inside, minimum diameter.Such a depth of groove is one which will co-operate with adjacent landsto increase heat transfer on the evaporatorating tube, is readilyobtainable by swaging a plain walled tube and contributes to turbulencein the tube bore.

DESCRIPTION OF THE DRAWING One tube which embodies the invention willnow be described with reference to the accompanying drawing in which:

FIG. 1 is a plan view of one end of a tube,

FIG. 2 is an axial cross-section through a profiled portion of FIG. 1.

FIG. 3 is a developed view of part of FIG. 1,

FIGS. 4A and 4B are graphs showing the performance of the tube shown inFIG. 1 as compared with a smooth walled tube.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the drawing, FIG. 1 shows thetube with a plain end portion 1 suitable for fitting into a conventionaltube plate and a specially profiled portion 2. The latter commences atthe plain portion with eight start helical grooves 3, with interveninglands 4 which are initially right-handed. These grooves proceedhelically along the tube wall for about a quarter of a wavelength andthen smoothly reverse hand, to proceed left handed for a further quarterof a wavelength reverse hand again and so on. The wavelength of thepart-helical turns is about four tube diameters. A wave-length may bevaried between 1 and 0 tube diameters without departing from theinvention. The groove depth d is also related to the tube insidediameter Die the minimum diameter and it is preferred that a ratio ofgroove depth d to tube inside diameter equal to 0.15 be adhered to.

With the tube employed with its axis vertical, the pitch of the groovesmust not be longer than specified, or else a condensate drainage path onthe groove would build-up a condensate film to an unwelcome thickness.The reversal of the helical grooves on a short pitch breaks up thecontinuous path length into a number of short lengths thus, condensatedraining down any land on the outside of the tube will intersect agroove sooner or later. However, the pitch is not too short such that itwill impede run-off of condensate.

It will be seen from FIG. 3 that the fluid flowing near the bore wallwill follow a very confused part helical flow pattern, the fluid nearthe tube wall continually meeting helical grooves of changing hand sothat a high degree of turbulence results, yielding a good heat trans fercharacteristic. The outside of the tube has a continuous longitudinalpeaks and troughs 6 complementary to those in the bore; the outsidetroughs 6 provide channels for the run off of condensate with gathers inthe grooves leaving the peaks exposed to ambient vapour.

In practice the groove depth is linked to the inside diameter of thetube owing to the fact that the greater the groove depth the greater thepressure drop and this becomes the limiting factor in case of the waterflowing through the tube bore. To this extent the ratio between groovedepth and tube internal diameter lies between 0.02 and 0.2 and ispreferably equal to or less than 0.15.

The pattern of grooves shown in the drawing is formed preferably byusing a planetary swage tool, the tool comprising a swage .ring fromwhose inner diameter protrude eight hard steel balls at equal spacingaround the ring. The swage ring is engaged with the tube periphery anddrawn axially along it; at the same time the required degree of rotaryoscillation is imparted to the swage (or the work) to produce helicalgrooves as shown. This will produce the characteristic wrinkled tubewall as opposed a ribbed wall.

The respective dimensions of a typical tube were as follows:

Helix lead 3.5 inches Starts 8 Of course the form of the grooves may bealtered within the ambit of the invention to suit differentapplications.

The performance of the above tube for heat transfer between waterflowing in the tube bore and steam condensing on the tube periphery wasas shown on the graphs FIGS. 4A, 4B. The invented tube is indicated infull iine and the equivalent performance of a copper smooth-walled tubeis shown in dotted line. The high degree of turbulence imparted to thefluid in the tube bore makes the tube of the invention especially suitedto pass a highly viscous fluid such as crude oil. In this case. the oilwithin the tube would be receiving heat from a heating fluid passed overthe tube surface. The tube is equally suited to use with less viscousoils and liquors used in industrial plant which are required to bepreheated, prior to chemical treatment. by gas or other heating fluidpassed or flowed over the tube surface. Of course the helix lead may bevaried to give required dcgree of turbulence. Generally the more viscousthe fluid the shorter the helix lead and/or the greater the depth of thegrooves.

Condensing surfaces in accordance with the invention are amenable to usein a wide range of applications, notably in evaporator plants as usedfor desalination and in the food and chemical industries. ln the case ofdesalination such condensing surfaces may be employed with advantage forthe condenser tubes of a plant of the flash type and for the preheatertubes in plants of the multi-effect type. In the food and chemicalindustries such condensing surfaces may serve in similar roles in theconcentration of solutions, residues and similar products.

We claim:

1. A condenser having at least one vertical tube for condensing vapor onits outer surface. the improvement comprising the tube wall being ofsubstantially uniform thickness and shaped so as to exhibit a number ofparallel part-helical continuous lands and grooves in the tube bore toimpart turbulence to the coolant, and grooves and lands on the tubesurface to assist in run off of condensate, the hand of the helixchanging at intervals along the tube before the helix has completed acomplete turn, the pitch of the grooves (determined by half a wavelengthof the helix) being more than 0.5 but less than 5 times the inside, i.e.minimum, diameter of the tube.

2. A condenser as claimed in claim 1 in which the pitch is about twicethe tube internal diameter.

3. A condenser as claimed in claim I in which the ratio of groove depthto the minimum inside diameter of the tube lies between 0.02 and 0.2.

4. A condenser as claimed in claim 1 in which the ratio of groove depthto the inside diameter of the tube is equal to 0.15.

1. A condenser having at least one vertical tube for condensing vapor onits outer surface, the improvement comprising the tube wall being ofsubstantially uniform thickness and shaped so as to exhibit a number ofparallel part-helical continuous lands and grooves in the tube bore toimpart turbulence to the coolant, and grooves and lands on the tubesurface to assist in run off of condensate, the hand of the helixchanging at intervals along the tube before the helix has completed acomplete turn, the pitch of the grooves (determined by half a wavelengthof the helix) being more than 0.5 but less than 5 times the inside, i.e.minimum, diameter of the tube.
 2. A condenser as claimed in claim 1 inwhich the pitch is about twice the tube internal diameter.
 3. Acondenser as claimed in claim 1 in which the ratio of groove depth tothe minimum inside diameter of the tube lies between 0.02 and 0.2.
 4. Acondenser as claimed in claim 1 in which the ratio of groove depth tothe inside diameter of the tube is equal to 0.15.